Borophene shows highly anisotropic metallic behaviour (a) due to anisotropic structure (b). The optical properties of borophene are orientation-dependent (c). This provides opportunities for applications in display technologies, photovoltaics and flexible electronics.

Our work shows that the thermal conductivityκof MoS₂is largest among the three materials (Fig. a and b) due to largest Debye temperature. WS₂has stronger bonding (Fig. e) and lower anharmonicity (Fig. h), leading to a much largerκcompared to MoSe₂.

A new 2D material, borophene, has been grown successfully recently on single crystal Ag substrates. Three main structures have been proposed ( , and striped borophene). However, the stability of three structures is still in debate. Using first principles calculations, we examine the dynamical, thermodynamical and mechanical stability of , and striped borophene. Free-standing and borophene is dynamically, thermodynamically and mechanically stable, while striped borophene is dynamically and thermodynamically unstable due to high stiffness along a direction. The origin of high stiffness and high instability in striped borophene along a direction can both be attributed to strong directional bonding. Our work shows a deep connection between stability and strength, and helps researchers to estimate accurately the mechanical performance of 2D materials. GRAPHICAL ABSTRACT IMPACT STATEMENT

A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures.

Controlling heat transport through material design is one important step toward thermal management in 2D materials. To control heat transport, a comprehensive understanding of how structure influences heat transport is required. It has been argued that a buckled structure is able to suppress heat transport by increasing the flexural phonon scattering. Using a first principles approach, we calculate the lattice thermal conductivity of 2D mono-elemental materials with a buckled structure. Somewhat counterintuitively, we find that although 2D group-V materials have a larger mass and higher buckling height than their group-IV counterparts, the calculated κ of blue phosphorene (106.6 W mK⁻¹) is nearly four times higher than that of silicene (28.3 W mK⁻¹), while arsenene (37.8 W mK⁻¹) is more than fifteen times higher than germanene (2.4 W mK⁻¹). We report for the first time that a buckled structure has three conflicting effects: (i) increasing the Debye temperature by increasing the overlap of the p_z orbitals, (ii) suppressing the acoustic–optical scattering by forming an acoustic–optical gap, and (iii) increasing the flexural phonon scattering. The former two, corresponding to the harmonic phonon part, tend to enhance κ, while the last one, corresponding to the anharmonic part, suppresses it. This relationship between the buckled structure and phonon behaviour provides insight into how to control heat transport in 2D materials.